1. Field of the invention
The present invention relates to a velocity detecting apparatus and a velocity controlling apparatus in a seek circuit of an optical disk apparatus or the like. More particularly, the invention relates to a velocity detecting apparatus and a velocity controlling apparatus which enable a stable velocity detection operation and a velocity control operation free from influence of circuit constant or the like, in arrangement of digital circuit.
2. Description of the Related Art
A conventional optical disk apparatus is provided with seek means comprising a track pulse generator, which generates a certain number of track pulses every time a light spot focused on an optical disk crosses a track on the optical disk, and a velocity detector for detecting a moving velocity of the light spot from a frequency of the track pulses (see for example Japanese Patent Laying Open (Kokai) Application No. 2-33731).
The conventional optical disk apparatus of this type normally employs a so-called F/V converter, which is a circuit for converting the frequency of track pulses periodically obtained in accordance with the relative movement between the light spot and the tracks, into a voltage in order to detect a relative velocity between the light spot and the tracks on the optical disk.
FIG. 1 is a functional block diagram to show an example of main structure of the F/V converter. In FIG. 1, reference numeral 11 designates a mono multivibrator (MM), 12 an amplifier, 13 a resistor, 14 a condenser, IN an input signal, MMO an output of the mono multivibrator 11, and OUT an output signal of the amplifier 12.
FIG. 2 is a time chart to illustrate an operation of the F/V converter as shown in FIG. 1. Reference characters given for respective signal waves correspond to locations of the characters in FIG. 1.
As shown by MMO in FIG. 2, the mono multivibrator 11 is triggered at each leading edge of the input signal IN to provide an output of pulse with a constant width.
The output MMO of the mono multivibrator is amplified by a subsequent amplifier 12, and the amplifier 12 generates an output signal as shown by OUT in FIG. 2.
The F/V converter is, however, an analog circuit, an output of which may include considerable error elements risen from a dispersion of analog circuit constant and from an offset or a gain error of the amplifier.
Such errors may cause a problem of unstable seek velocity control in the F/V converter.
In addition, the seek velocity control loop itself is an analog circuit basically composed of a an F/V converter, a subtracter, and an amplifier, which is likely to cause errors in hardware and which is also unsuitable for stable velocity control.